Update asciidoctor-fb2 requirement from 0.7.0 to 0.8.0 (#71)

Updates the requirements on [asciidoctor-fb2](https://github.com/asciidoctor/asciidoctor-fb2) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor-fb2/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor-fb2/blob/main/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctor-fb2@v0.7.0...v0.8.0)

---
updated-dependencies:
- dependency-name: asciidoctor-fb2
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile

@@ -6,7 +6,7 @@ gem 'asciidoctor', '2.0.23'
 gem 'json', '2.9.1'
 gem 'awesome_print', '1.9.2'
 
-gem 'asciidoctor-fb2', '0.7.0'
+gem 'asciidoctor-fb2', '0.8.0'
 gem 'asciidoctor-epub3', '2.1.3'
 gem 'asciidoctor-pdf', '2.3.19'